In many instances of polymer technology and polymer applications, it is desirable to impart to a polymer article surface properties which are different from the properties of the bulk. Good examples include the treatment of textiles to make them either more hydrophilic or more hydrophobic by coating with thin films of either hydrophilic or hydrophobic polymers, including fluorinated oleophobic polymers and curing and crosslinking them on the fiber to achieve some permanence. While this will produce a homogeneous continuous film of definite thickness, permanence is usually poor because little or no covalent bonding exists between the polymer and the fiber, and by abrasion during repeated washing, dry cleaning and drying cycles the polymer film is easily removed.
Another method for modifying polymer-surfaces is grafting. Grafting is the covalent attachment of monomers or polymers on existing polymer chains, for instance solid polymer surfaces. This can be accomplished in numerous ways. For example, treatment with ionizing radiation (x-rays, electron beams) in the presence of oxygen of a polymer surface, followed by heat-treatment of the peroxidized polymer in presence of an appropriate vinyl monomer will give graft-modified surfaces. Another method to introduce peroxy groups onto a polymer surface is by ozone treatment. Treatment with a plasma (ionized gas) alone can oxidize and otherwise chemically modify a polymer surface, dependent on the nature of the plasma, and this method can be used to hydrophilize polyolefins in order for them to absorb printing ink.
Grafting onto solid polymers can also be carried out by first immersing the polymer in a monomer with an initiator for a specified time, followed by exposing the surface-swollen polymer to polymerizing conditions. Many variation of the mentioned processes have been described, all having as the objective the grafting of a vinyl polymer on a preformed polymer substrate.
There are several major drawbacks to all these processes and products; for instance polymerization can occur not only on the surface, but also in the liquid polymerization medium. Also, because the polymerization depends on active centers which decompose at different times, inhomogeneous surfaces result because of preferential absorption of monomer into already modified surface regions. In cases where exact depth-control and uniformity of the grafted region is important, such as for example in the surface modification of contact lenses, such uncontrollable grafting reactions are not acceptable. On the other hand, if, to reduce inhomogeneities, grafting is carried out for a short time only, the grafted surface regions are too thin and in many applications the surface effect soon wears off.
Although free-radical grafting of vinyl-monomers is by far the most commonly used method for grafting onto polymer surfaces, if these polymers contain suitable reactive groups, other reactions can be utilized; polyvinyl alcohol or cellulose can for instance be grafted with isocyanato compounds, polydienes with mercaptans, and oxirane groups containing polymers with amines and mercaptans. In most practical cases however the polymer one wishes to modify is an inert, non-reactive polymer.
Yet another method of surface-modification, this one by formation of an interpenetrating polymer network (IPN) in which a second polymer interpenetrates a first one without covalent bonding between both, has been described in the literature, for example in U.S. Pat. No. 4,423,099. These methods too have the disadvantage of having swelling steps involved which in applications like contact lenses lead to unacceptable surface distortions.
In the contact lens field, surface properties are important because they affect deposit-formation and wettability and thereby the comfort of the lens for the wearer. Lack of wettability is an especially serious problem in contact lenses made from silicone rubber, which because of its very high, oxygen permeability would otherwise be an ideal contact lens material. Many of the mentioned processes of surface-grafting and irradiation have been used to overcome this wettability problem and either suffer from a lack of permanence or from poor optical quality, as is indicated by the fact that a generally accepted silicone rubber soft contact lens has not yet been produced. Relevant patents in this area include: U.S. Pat. No. 3,925,178, which describes surface treatment by water-vapor subjected to an electrical discharge; U.S. Pat. No. 4,099,859 describes grafting a hydrophilic polymer onto a silicone-rubber contact lens by coating the lens with a hydrophilic monomer followed by exposure to UV; U.S. Pat. No. 4,229,273 describes grafting of an acrylic polymer onto a silicone rubber lens pre-irradiated in the presence of oxygen; U.S. Pat. No. 4,168,112 describes the formation of a poly-electrolyte complex on the surface of a contact lens in order to render it more hydrophilic and comfortable to wear; U.S. Pat. No. 4,217,038 describes the formation of a glass-coating on a silicone contact lens to improve wettability; U.S. Pat. No. 4,409,258 describes a hydrophilizing treatment of contact lenses by bombardment with nitrogen and oxygen containing gas-ions; U.S. Pat. No. 4,388,164 describes coating the surface of a silicone rubber with a thin metal film by vacuum decomposition in the stretched state; U.S. Pat. No. 4,332,922 describes hydrophilization of silicone contact lenses by a gas-discharge; U.S. Pat. No. 4,143,949 describes a hydrophilic coating onto a hydrophobic contact lens by radiation induced polymerization; U.S. Pat. No. 4,311,573 describes hydrophilization of a hydrophobic polymer by ozone treatment, followed by a grafting of vinyl monomers through decomposition of the formed peroxy groups; U.S. Pat. No. 4,652,672 describes surface hydrophilization of polysiloxane contact lens materials by inclusion of N-alkenoyl trialkylsilylaminate, 1-12 parts by weight, into the monomer mixture and hydrolysis after polymerization.
Although many of these reactants are successful in increasing the wettability of a hydrophobic polymer, the effect is generally limited to a thin surface region and therefore is not permanent.
It would be desirable to have an integral, hydrophilic surface layer completely covering the underlaying hydrophobic polymer in such a way, that no hydrophobic groups can penetrate to the surface. It is, however, difficult to produce such hydrophilic/hydrophobic laminates with good adhesion, especially if on immersion in water one of the polymers swells a much higher degree than the other.
In copending patent application Ser. No. 250,199 a process is described, in which a mold is coated with a hydrophilic polymer, bearing groups which are capable of copolymerizing with the reactive groups of the polymerizing mixture to be molded, and which hydrophilic polymer is transferred by grafting during the polymerization onto the surface of the molded part, forming an integral and continuous skin. Because the reactive polymer coating is a hydrophilic polymer that skin on the surface of the molded part will be completely hydrophilic and in no way resemble the underlying polymer. Because of the covalent bonding between both polymers and because of the defined thickness of the grafted polymer film, the surface is very abrasion resistant and durable, while at the same time precisely replicating the mold surface. It has further been discovered, that methacrylate functional hydroxyethyl cellulose is especially well suited to act as the hydrophilic polymer used for coating the mold, and hydroxyethyl cellulose bearing urethane-connected acrylate- or methacrylate groups is the object of this invention.
In contrast to conventionally grafted surfaces, where the grafted surface polymer chains are essentially dangling chains, attached to the substrate at one end, the grafted polymer chains of this invention are multiply attached to the substrate at sites distributed along their whole length.
Furthermore, and also in contrast to conventionally grafted surfaces where the grafted surface layer is formed in an open medium--air, nitrogen, solvents--and therefore does not exactly conform to a desired surface shape or surface quality, the grafted hydrophilic hydoxyethyl cellulose coating of this invention is preshaped, conforming to a given substrate, and therefore the final product composite precisely replicates that substrates configuration and surface quality.
Specifically, this invention describes hydroxyethyl cellulose, whose hydroxy groups have been partially reacted with a vinyl-unsaturated isocyanate, preferably an isocyanatoalkyl acrylate or methacrylate.
U.S. Pat. No. 4,565,857 discloses the reaction product of cellulose-acetate-butyrate (CAB), containing 4.3% residual hydroxy groups, with 2-isocyanato-ethyl methacrylate (IEM) as UV-curable protective coating; U.S. Pat. No. 3,782,950 describes the reaction product of hydroxypropyl cellulose with IEM, also for use as a protective coating. U.S. Pat. No. 4,605,622 describes photocurable polysaccharides of 300-100000 MW, bearing at least two unsaturated groups per molecule, obtained by reaction with glycidyl (meth)acrylate, itaconic anhydride or maleic anhydride.